Method of treating diesel fuel



Patented June 30, 1942 F METHOD OF TREATING DIESEL FUEL Stewart H. Hulse and John 0. Collins, Westfield, N. J., assignors to Standard Oil Development Company, a corporation of Delaware No Drawing. Application March 28, 1940, Serial No. 326,360

7 Claims.

This invention relates to the preparation of oil compositions increased in sulfur content by stabilized sulfur-containing organic compounds that improve characteristics of the compositions without imparting to them undue corrosiveness. More particularly, this invention is concerned with the modification of fuels for high speed compression-ignition engines of the Diesel type to improve their ignition qualities while their sulfur content is made unobjectionable,

For a Diesel type engine, the ignition quality of the fuel is considered to depend mainly on the time lag between injection of the fuel and spontaneous ignition of the fuel in the engine during the compression stroke. A reduction in this time lag or ignition delay period, such as obtained with increased concentration of normal cetane in a fuel, improves the performance of the engine, and this improvement with respect to any fuel can be evaluated in terms of cetane number increase according to the method discussed in the S. A. E. Journal of June, 1936, page 225.

It has been found that in refining Diesel fuels to eliminate odoriferous mercaptans, the ignition quality and corrosion tendencies of the fuel may be affected adversely or beneficially. Recently, methods have been proposed for treating a sour Diesel fuel with a large excess of sulfur or added quantities of mercaptans, using the I doctor sweetening process, in which an aqueous solution of sodium plumbite is used in essentially stoichiornetric proportions to form lead mercaptides with the m'ercaptans' in the absence of air. It has been pointed out that by these methods, organic disulfides and higher poly'sulfid'es are incorporated into the fuel with a resulting improvement in the cetane number of the fuel. But these methods are difficult to control, and unless they are employed with exactness to a very restricted degree, the resulting product is objectionably unstable and corrosive. Also, it has been found that when the doctor sweetenin treatment is restricted to avoid the formation of unstable products, only a very small improvement, if any, is made in the ignition quality of the fuel.

A primary object of the present invention is to provide a method for converting organic sulfur compounds having low ignition accelerating action and those which are objectionably odoriferous or corrosive, into more stable and beneficial oil components.

By using the method of the present invention, a hydrocarborifuel adapted for Diesel engine? in distillation and viscosity characteristics and containing a known amount of mercaptans may be treated with definite amounts of sulfur to change the mercaptans into more stable and non-corrosive organic sulfur compounds which are highly beneficial to the ignition quality of the fuel.

A procedure illustrating one phase of this invention is broadly as follows:

A Diesel fuel, such as a gas oil fraction having a substantial mercaptan content is mixed in an agitator of suitable type, either batch or continuous, with a determined quantity of reactive sulfur, such as elementary sulfur, and an alkaline suspension of a heavy metal sulfide, such as of lead sulfide. The mixture is advantageously agitated by air blowing at ordinary temperatures. Usually the reaction is finished after about one-half to two hours of agitation. The amount of sulfur added is determined primarily by the quantity of mercaptan sulfur together with the amount of initially present reactive sulfur in the oil, and sufficient free sulfur is added to convert the mercaptans into the desirable stable organic sulfur compounds, which contain a higher proportion of sulfur than the mercaptans or even their disulfide derivatives. Oxygen derived from the air used for agitation, or from any other source, is not necessary, but is desirably injected into the reaction mixture to promote the reaction. Also, oxygen in oxidizing agents may be used to accelerate the reaction.

By means of the mercury test, certain types of sulfur compounds present in an oil can be determined in terms of mercury number, which is numerically equal to the milligrams of the active sulfur in these compounds per ml. of the oil, meaning by active sulfur, that which apparently is present in the effective ignition accelerating compounds. This test is conducted by shaking a measured quantity of the oil with clean metallic mercury in a bottle, then holding the bottle closely to a standard source of light so that the opacity of the oil content, as increased by dispersed metallic sulfide, can be measured in comparison to the opacity produced by known concentrations of the sulfide in the same kind of oil.

Experimental data show that when dealing with a naturally sour gas oil, and treating this oil with controlled amounts of sulfur with admixed lead sulfide, so that the mercury number of the oil is increased, there is a direct relationship between the mercury number and cetane ratin of the product as determined in a C. F. R. Diesel engine by comparison with standard reference fuels and obtaining accurate results which check to within less than one cetane number. Thus:

Mercury No. (active sulfur 32 3 rug/100 ml.)

Mercury No.

(active sulfur g mg./100 ml.)

Oil No. 5 5 48 From this it is evident that the organic sulfur compounds formed by the treatment were relatively higher boiling than the corresponding mercaptans, since in this case they tended to concentrate in the residuum upon distillation of the treated fuel.

In each of the foregoing experiments, the Diesel fuel product improved in ignition quality was non-corrosive to copper as determined by the A. S. T. M. copper strip corrosion test, in which a bright strip of copper is exposed to the fuel tested for at least three hours at 122 F. This shows that the improvement of the fuel, as judged by increase in mercury number or cetane number, did not cause the fuel to be corrosive to copper, as would be the case if the fuel were treated by a method which forms unstable and corrosive organic sulfides. This fact is forcefully demonstrated by the following experiment:

Samples of West Texas gas oil having the inspection analysis shown in the following tabulation were treated by two methods using the same amount of excess sulfur, one method using a doctor solution and the other method using an alkaline suspension of a catalytic heavy metal sulfide in accordance with the present invention:

Rerun, sodium carbonate washed, West Texas gas oil Gravity, A. P. I 362 Copper numbermg./100 ml '72 Reactive sulfurmg./100 ml 2 Distillation, F.:

I. B. P 389 456 50% 512 90% 578 F. B. P 617 Preliminary experiments indicated that the addition of 160 mg. of sulfur per 100 ml. of the gas oil was close to the maximum amount of sulfur which could be added if the lead sulfide and sulfur (P108 and S) treatment were employed, without producing a product which would be corrosive to copper in the A. S. T. M. test. Accordingly, this amount of sulfur was used in treating the gas oil by each of the two methods; and the following tabulation presents the data on the corrosion characteristics and cetane values of the oils in question:

It is to be noted from the above data that the catalytic metal sulfide treatment is capable of making a greater improvement than the doctor sweetening process in the cetanenumber of the fuel, when the same amount of free sulfur is used. With respect to corrosion, the lead sulfide treatment produced a decidedly superior product which was capable of withstanding a temperature of 122 F. for ten times as long a period as that endured by the doctor sweetened product. With reduction in the amount of sulfur used in the doctor sweetening process to obtain a noncorrosive product capable of passing the A, S. T. M. test, the product obviously is given a still lower cetane number.

Thus, it is quite evident that the method of the present invention transforms the free sulfur and unstable corrosive organic sulfur compounds into more stable compounds which, nevertheless, are highly active for promoting the ignition qualities of a Diesel fuel. It is also evident that the stable compounds formed contain a much higher proportion of sulfur than mercaptans or their disulfide derivatives, for if only disulfides were formed, there would have been no increase in mercury number.

The oil stocks involved in the foregoing experimental work were gas oil distillates which naturally contained sufiicient mercaptans to permit this demonstration without addition of mercaptans from an outside source. Of course, the treatment is not limited to such oils but may be used in preparing improved oil compositions from oils which initially contain only a small percentage of mercaptans, or even none at all.

In the practice of this invention to produce improved Diesel fuels, it has been found convenient to start with an oil containing suflicient mercaptans to give the oil a copper number of at least 10, the copper number being the measure in milligrams of mercaptan sulfur per cc. of oil, this quantitative determination being made by volumetric titration of an oil sample with a standardized ammoniacal copper sulfate solution. As the COpper number is equivalent to the concentration of mercaptan sulfur, it can be judged therefrom what quantity of free sulfur is needed for reaction with the mercaptans to obtain a given mercury number. Thus, in general, the theoretical proportion of reactive sulfur to be used for complete reaction with the mercaptans should be at least equal to one-half the copper number, and in accordance with experimental results, stabilized organic sulfur compounds that permit an oil to satisfactorily pass the corrosion test are formed when from 1 to 6 atoms of sulfur are reacted with each 2 molecules of mercaptans.

The oils are treated with a pre-determined quantity of sulfur in alkaline solution in which is suspended a heavy metal sulfide to catalyze the reaction to obtain a product having a mercury number of at least 20, the copper number being reduced correspondingly. When it is desired to avoid undue corrosiveness of the fuel, it has been found preferable to regulate the amount of added sulfur and mercaptan sulfur content so that the mercury number of the product does not substantially exceed about 200 as a maximum, but a greater increase of the mercury number may be obtained for further increase of cetane number, if desired. Thus, it has been found that fuels with very satisfactory corrosion avoidance are obtained when the oil treated contains from to about 75 mg. of mercaptansulfur per 100 cc. and from 20 to about 200 mg. of reactive sulfur is added per 100 cc. of the oil for the treatment.

When the oil to he used initially contains some corrosive sulfur reactive with mercaptans in the form of elementary sulfur, hydrogen sulfide, or polysulfides, such sulfur may also become fixed in the reaction; therefore, to avoid an excess of sulfur which would lead to corrosiveness beyond tolerated limits, account should be taken of the initial corrosive sulfur content so as to keep the mercury number duly within bounds. However, even an oil which has too high a mercury number may be given a further treatment using the present process with added sour oil or mercaptens to be brought back within the desired limits.

The reactive sulfur for the reaction may be conveniently added in the form of an aqueous solution of an alkali metal sulfide or polysulfides, especially with sodium as the alkali metal. Solutions of sulfur in oil are also suitable.

The catalytic lead sulfide is, in general, pre formed, but may be formed in the body of the oil or in aqueous solution by reaction of hydrogen sulfide with lead oxide or a water-soluble salt of the metal, before the reaction or in other suitable ways. Although lead sulfide has been found a most suitable catalyst for the reaction, sulfides of other multivalent elements or heavy metals, such as copper, cadmium, arsenic, or phosphorus may be used. The amount of the catalytic heavy metal sulfide required is relatively small in proportion to the reactants, for it is not necessary to have the metal sulfide present in amounts sufficient to convert the mercaptans into mercaptides, and substantially all the heavy metal comound present in the reaction mixture should exist essentially and mainly in the form of the sulfide. Experiments indicate that one molecule of the heavy metal sulfide is capable of converting many more than two molecules of mercantans into the stable organic sulfur compounds and that the heavy metal sulfide persists unchanged the product, hence, although the mechanism of the process is somewhat obscure. the heavy metal sulfide may be regarded as acting catalytically to cause condensation of the mercaptans with reactive sulfur, fixing of reactive sulfur with mercaptans into stable organic compounds, and splitting off of labile sulfur from unstable polysulfides.

In order to obtain any desired higher increase in. cetane rating of a fuel by formation of stable organic sulfur compounds in use, the mercaptan content of the oil may be increased by adding suitable mercaptans derived from an outside source, by blending with another oil of higher mercaptan content, or, very suitably, by admixing spent caustic solution resulting from soda Washing of cracked naphthas, heating oils, or

other distillates. Specific mercaptans which are considered to be particularly useful for the reaction are alkyl mercaptans containing from 2 to 6 carbon atoms in each alkyl radical, and more particularly those having branched alkyl radicals, e. g., isopropyl mercaptan, tertiary butyl mercaptan, and iso-amyl mercaptan. However, alkyl aryl, aryl, alkaryl, or aralkyl mercaptans may also be employed at least to some extent.

It is to be observed that, in general, the desired stable blending agents having a high content of fixed sulfur are obtained by the reaction of corrosive sulfur-containing substances, such as free sulfur or corrosive polysulfides with mercaptans or organic compounds that readily form mercaptans in the presence of the reactive or labile sulfur in such corrosive substances with intervention of a heavy metal sulfide as a catalyst, in an alkaline medium, and preferably with an oxidation agent as a promoter. Thus, in place of mercaptans and free sulfur, reactive organic compounds which readily form mercaptans and reactive polysulfides under the conditions of the reaction may be used.

Every type of hydrocarbon base oil having suitable fuel value and physical characteristics for Diesel engines may be employed for improvement with the stabilized organic sulfur compounds prepared according to the present method. In most general use, a petroleum distillate boiling above the gasoline boiling range, such as a gas oil fraction, constitutes the major proportion of the fuel, because the fuel injection systems of Diesel engines function properly with a fuel having a viscosit of about 30 to Saybolt seconds at F. For example, a petroleum oi]. boiling in the range of 375 F. to 700 F. has satisfactory viscosity characteristics for a Diesel fuel. The oil fractions used may be derived from any petroleum base stock, or mixed stocks, may be obtained by any method of r fining, such as straight distillation, thermal or catalytic cracking, hydrogenation, solvent treating, etc., or any combination of such treatments.

While the method of this invention affords a way of transforming corrosive sulfur into more stable or organic sulfur compounds which are particularly advantageous ignition accelerating agents in Diesel fuels, it also makes available a method of rendering organic sulfur compounds more stable and less corrosive to copper and similar metals commonly occurring in modern engine parts as in alloy bearings. Many high- 1y sulfurized organic compounds which are predisposed to be corrosive to copper including polysulfides, substituted aromatic sulfides. e. g. alkyl phenol sulfides and their salts, have been found to be valuable oil blending agents for improving film strength, inhibiting oxidation, for lowering sludging, and for preventing varnishlike formation on engine parts. By use of the present method of fixing corrosive sulfur into a more stable form, such compounds are likewise corrected to prevent excessive corrosiveness to copper and similar metals, i. e., particularly metals like cadmium, lead, nickel, and silver, which are used in modern alloy bearings of internal combustion engines.

For most purposes, the stabilized organic sulfur compounds obtained by the present procedure are generally to be blended with an oil so that they give the oil an added sulfur concentration ranging from about 0.01% to about 1.5% by weight anad preferably from 0.02% to 0.4% by weight. These increases in active sulfur con- It will be appreciated that the present inven tion is not restricted to the formation of the stabilized organic sulfur compounds only in the oil which is to be used as a final product and that these compounds may be prepared in one medium, then blended with any suitable hydro carbon oil base. Moreover, it is not intended to limit the use of stabilized sulfur compounds entirely in Diesel fuels, because stabilized organic sulfur compounds are considered useful also as blending agents in motor oils, gear lubricants, industrial oils, greases, and other types of oil compositions which require a restricted amount of corrosiveness. Thus, in general, they are to be used in mineral carbonaceous or petroleum oils boiling above the gasoline range and having viscosities ranging upwardly from about 30 Saybolt seconds at 100 F. They may also be used together with other addition agents which serve their specific functions, as for example, oiliness agents, such as fatty compounds, soaps, thickeners, oxidation inhibitors, dyes, corrosion inhibitors, viscosity index improvers, pour point depressants, or agents which additionally impart the same improvements to the oil as do the stabilized organic sulfur compounds.

It will be understood that the foregoing examples and theories of mechanism are illustrative and that various obvious changes may be made within the scope of the appended claims.

We claim:

1. A method for improving sour Diesel fuel petroleum distillates which comprises adding to a sour petroleum Diesel fuel distillate oil con taining at least mg. of mercaptan sulfur per 100 ml. of said oil, lead sulfide, an alkali and reactive sulfur, the reactive sulfur being added in a proportion to form by reaction with the mercaptans stabilized organic compounds containing from to about 200 mg. of active sulfur per 100 ml. of the oil and reacting the mercaptans with the reactive sulfur in the presence of the lead sulfide, substantially all combined lead persisting as a catalyst throughout the reaction to form stabilized organic sulfur compounds which satisfactorily permit the product to pass the A. S. T. M. copper strip corrosion test in which the copper strip is exposed to the product for three hours at 122 F.

2. The method of improving the ignition quality of a hydrocarbon Diesel fuel which comprises reacting free sulfur with mercaptans in a sour Diesel fuel distillate containing admixed lead sulfide and an alkaline medium, the reactive free sulfur being present in a proportion from 20 to about 200 mg. per 100 ml. of distillate, mercaptan sulfur being present in a proportion of from 10 to about '75 mg. per 100 m1. of the distillate, the lead sulfide present containing substantially all combined lead for initiating said reaction and persisting as a catalyst throughout the reaction to form organic sulfur compounds which have ignition quality improving effectiveness and suificient stability to permit the product to pass the A. S. T. M. copper strip corrosion test in which the copper strip is exposed to the product for 3 hours at 122 F., and blending at least a portion of the product containing said stable organic sulfur compounds with additional hydrocarbon Diesel fuel.

3. A method for improving a Diesel fuel petroleum oil which comprises reacting free sulfur with mercaptans in a sour petroleum Diesel fuel oil containing at least 10 mg. of mercaptan sulfur per ml. of oil in the presence of an alkali and lead sulfide, said lead sulfide containing substantially all combined lead in the oil for initiating the reaction and persisting as a catalyst throughout the reaction, said free sulfur being present in a proportion of from 20 to about 200 mg. per 100 ml. of the oil, agitating the reaction mixture with air until the mercaptans are substantially converted by reaction with the free sulfur into organic sulfur compounds which are effective ignition promoters but which are sumciently stable to permit the oil containing them to satisfactorily pass the A. S. T. M. copper strip corrosion test in which the copper strip is exposed to the product for 3 hours at 122 F.

4. The method of improving a Diesel fuel distillate which comprises reacting mercaptans in a sour hydrocarbon Diesel fuel oil containing at least 10 mg. of mercaptan sulfur per 100 ml. of said oil with reactive sulfur present in a concentration of from 20 to about 200 mg. per 100 ml. of the oil and in the presence of lead substantially all combined in lead sulfide to initiate the reaction and persist throughout the reaction as a catalyst with an alkali medium, agitating the distillate during the reaction by blowing with air, and continuing the reaction until the mercaptans are substantially converted into organic sulfur compounds having ignition quality improving effectiveness and sufficient stability to permit the distillate in which they are present to pass the A. S. T. M. copper strip corrosion test in which the copper strip is exposed to the prodnot for 3 hours at 122 F.

5. The process as described in claim 4 in which alkyl mercaptans having from 2 to 6 carbon atoms in the alkyl radicals are present in said sour distillate for reaction with the reactive sulfur.

6. The process as described in claim 4 in which a corrosive polysulfide is admixed with said sour distillate to furnish at least a portion of said reactive sulfur.

7. A process as described in claim 4 in which 2 molecules of mercaptans reacted are directly condensed with from 1 to 6 atoms of the reactive sulfur.

STEWART H. HULSE. JOHN O. COLLINS. 

